A supervisor in a bank want to measure the service time per customer by a teller. By 8:30 there were already 10 customers in line waiting for the teller. Then at exactly 8:00 the teller started serving the first customer. Once the first customer is finished, the second customer is served directly and so on. The supervisor immediately noted what time each customer was. finished serving. Following are the notes for the 7 customers above when they are completed: 8:05; 8:07; 8:15; 8:20; 8:24; 8:29; 8:35.

Based on these 7 customers calculate the standard deviation of the service time (in minutes).